1. Field of the Invention
The present invention relates to an antenna device for millimeter wave band or the like comprising a dielectric lens and a primary radiator, and also relates to a transmit-receive unit using the antenna device.
2. Description of the Related Art
Radar for a vehicle, using the millimeter wave band, for example, radiates a highly directed radar beam forward or rearward of the vehicle, receives waves reflected from a target such as another vehicle traveling in front of or behind the vehicle, and determines the distance to the target and its speed relative to the vehicle itself based on time delay, frequency difference, and the like, between the radiated and received signals. In a millimeter wave radar of this type, when a scan is to be conducted across a small angular range, the radar need only to radiate the transceiver beam in a fixed direction. In contrast, when scanning is to be conducted across a large angular range, the radar must change the direction of the beam while maintaining a high directivity so as to maintain high gain without reducing the resolution.
Accordingly, in a conventional millimeter wave antenna device, such as that shown in FIG. 7, a dielectric lens 2 and a primary radiator 1 constitute a single antenna device, and the direction of the beam is changed by changing the relative position of the primary radiator 1 with respect to the dielectric lens 2. In FIG. 7, reference numerals 1a, 1b, and 1c simultaneously represent three positions during the beam scanning of a single primary radiator. When the primary radiator 1 is at position 1a, the beam is formed as shown by Ba; when the primary radiator 1 is at position 1b, the beam is formed as indicated by Bb; and when the primary radiator 1 is at position 1c, the beam is formed as indicated by Bc. FIG. 8 shows an example of changes in the beam depending on the position of the primary radiator 1.
Since the above-mentioned dielectric lens is a rotationally symmetric body having its central axis as its center, a focal point is normally created on this central axis (hereinafter termed the "optical axis"), and the resulting beam is most focused when the phase center of the primary radiator is at the focal position. In the example shown in FIG. 7, the beam Bb, formed when the primary radiator is at the position indicated by 1b, is focused and is obtained with high gain. The further the phase center of the primary radiator deviates from the focal point, the wider the beam (half-value angle), and the weaker the emission, with a consequent reduction in the gain. Accordingly, in general, the phase center of the primary radiator is moved along the plane (hereinafter termed the "focal plane") perpendicular to the optical axis passing through the focal point, and tracking is performed keeping the beam as focused as possible, thereby preventing a reduction in gain.
However, when there is a need to widen the angle of the beam scanning, the displacement of the primary radiator increases, and is inclined greatly with respect to the optical axis of the dielectric lens. As a result, the open efficiency of the dielectric lens decreases. In addition, the effects of aberration increase, greatly changing the gain of the antenna. Furthermore, even when the angular range of the beam scanning is relatively small, when a more uniform gain is required, there is still the problem of changes in gain due to the displacement of the primary radiator.